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Chapter 3Datums Learning Objectives After studying this chapter, you will be able to:❍ Explain the purpose of datums.❍ Defi ne common terms related to datum features.❍ Interpret information provided by datum feature symbols.❍ Describe methods for representing datum surfaces on drawings.❍ Explain how to establish datum reference frames.❍ Identify the degrees of freedom of a part.❍ Explain how to establish datum targets.❍ Identify datum target points, areas, and lines.❍ Describe methods for representing datum axes on drawings.❍ Describe methods for representing datum center planes on drawings. Technical TermsActual mating envelope NormalCoaxial Order of precedenceCoaxial datum features Primary datumCoplanar surfaces Primary datum planeDatum axis RotationalDatum feature Secondary datumDatum feature simulator Secondary datum planeDatum feature symbol Simulated datumDatum plane Simulated datum axisDatum reference frame (DRF) Simulated datum planeDatum reference order Tangent planeDatum target line Tertiary datumDatum target point Tertiary datum planeDatum targets Three degrees of rotationDatums Three degrees of Degrees of freedom translationHigh points TranslationalMovable datum targets Datums are considered to be theoretically perfect planes, surfaces, points, lines, axes or a combination thereof derived from the datum feature simulator, which is described in this chapter. 77